compareBottom

Compare the bottom of the queue with doc. This will
only invoked after setBottom has been called. This
should return the same result as FieldComparator.compare(int,int)} as if bottom were slot1 and the new
document were slot 2.

For a search that hits many results, this method
will be the hotspot (invoked by far the most
frequently).

compareValues

Returns -1 if first is less than second. Default
impl to assume the type implements Comparable and
invoke .compareTo; be sure to override this method if
your FieldComparator's type isn't a Comparable or
if your values may sometimes be null